Powder metallurgy methods and compositions

ABSTRACT

The present invention provides metal powder compositions for pressed powder metallurgy and methods of forming metal parts using the metal powder compositions. In one embodiment, the metal powder composition according to the invention includes a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof, a lubricant and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions. During a “delubing” step, the organic acid reacts with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes into a base metal or a metal-carbide during sintering.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to methods and compositions for use in pressed powder metallurgy.

2. Description of Related Art

In pressed powder metallurgy, a substantially dry metal powder composition is charged into a die cavity of a die press and compressed to form a green compact. Pressing causes the metal powder particles in the metal powder composition to mechanically interlock and form cold-weld bonds that are strong enough to allow the green compact to be further processed. After pressing, the green compact is removed from the die cavity and sintered at a temperature that is below the melting point of the major metallic constituent of the metal powder composition, but sufficiently high enough to strengthen the bond between the metal powder particles, principally through solid-state diffusion. Some metal powder compositions include minor amounts of other metals and/or alloying elements that melt during sintering to facilitate liquid phase sintering of the non-melting major constituent of the metal powder composition. This increases the bonding strength between the metal powder particles and typically increases the final density of the sintered part.

In most pressed powder metallurgy applications, it is necessary to add a lubricant to the dry metal powder composition before it is pressed to form the green compact. The most commonly used lubricants in pressed powder metallurgy are ethylene bis-stearamide wax and zinc stearate, but other lubricants are also sometimes used. The lubricant helps the individual metal powders flow into all portions of the die cavity, allows for some particle to particle realignment during pressing and also serves as a release agent that facilitates removal of the green compact from the die cavity after pressing. The least amount of lubricant necessary to obtain good flow and release is used.

The lubricant is conventionally removed from the green compact by gradually heating the green compact at a relatively low heating rate (e.g., ˜15°F./min) until the lubricant melts, boils and/or decomposes. This “delubing” is typically accomplished during an initial heating or preheating stage at the beginning of the sintering process. This can be accomplished in a batch furnace or in a continuous furnace. In a continuous furnace, the green compact is placed on a conveyor that moves the part slowly into and through a sintering oven. The slow movement of the conveyor allows the temperature of the green compact to increase at a slow rate, allowing the lubricant to melt and then boil and then gas off. Most of the remaining lubricant residue is decomposed and burned out as the temperature of the green compact increases. Some small quantity of the lubricant may diffuse into the base metal and contribute carbon to the final part. The lubricant is completely removed from the green compact at a temperature that is substantially lower than the final sintering temperature. In a batch furnace, the temperature is gradually increased to remove the lubricant prior to sintering that may be programmed to run at different conditions.

To maximize the opportunity for the individual metal particles to bond to each other, it has long been the practice to sinter the green compact at a peak sintering temperature for a significant amount of time, typically on the order of 30 minutes or more. Allowing the part to soak or dwell at the peak sintering temperature for this period of time is believed to increase the likelihood that individual metal particles will bond via solid-state diffusion. The slow movement of the conveyor or the temperature profile in a batch furnace insures that the green compact receives a lengthy soak or dwell time in the hot zone of the sintering oven.

Ideally, the sintered density of a final part would be 100% of the theoretical density of the metallic constituents of the metal powder composition used to form the part. However, the sintered density of parts formed from most metal powder compositions does not approach 100% of theoretical density. Using conventional carbon or low alloy steel metal powder compositions and pressed powder metallurgy methods, a sintered density of about 93% to 94% of theoretical density can be achieved.

BRIEF SUMMARY OF THE INVENTION

The present invention provides metal powder compositions for pressed powder metallurgy and methods of forming metal parts using the metal powder compositions. In one embodiment of the invention, the metal powder composition comprises a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof, a lubricant and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions. Preferably, the primary metal particles comprise iron, which may be alloyed with other metals/elements such as, for example, carbon, copper, manganese, phosphorus, silicon, sulfur, nickel, chromium, bismuth, cobalt, niobium, molybdenum, tungsten, tin, aluminum and titanium. The liquid phase forming materials are preferably selected from the group consisting of Fe—C—Mn, Fe—C, Fe—C—Si, Fe—Mn, Fe—P, Fe—S, Co—C, Mo—C, Mn—C, Ni—C, Fe—B and Fe—Cr, or precursors thereof selected from the group consisting of graphite, ferro phosphorous, copper phosphorous, boron, silica, manganese sulphide, manganese, silicon, phosphorous, sulfur, boron, chromium, cobalt and/or molybdenum.

A first method of forming a metal part according to the invention comprises: (i) providing a metal powder composition comprising a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof, a lubricant and an organic acid; (ii) placing the metal powder composition within a die cavity; (iii) applying pressure to the metal powder composition contained within the die cavity to form a green compact; (iv) heating the green compact in a non-oxidizing atmosphere to delube the metal powder composition and cause the organic acid to react with an oxide of a metal on the primary metal particles and form an organic metal salt; and (v) heating the delubed green compact to a peak sintering temperature at a heat up rate of 60° F./min or higher in a non-oxidizing atmosphere to decompose the organic metal salt into a base metal and/or a metal carbide and form the metal part. The conversion of the metal oxide on the surface of the primary metal particles to an organic metal salt during the delubing step creates a “clean” surface on the primary metal particles that is receptive to both liquid phase bonding and subsequent diffusion bonding. The rapid heating rate during the sintering step ensures that the liquid phase formers have adequate time to create liquid phase bonds between the primary metal particles before the constituents of the liquid phase diffuse into the particles.

In a second embodiment of the invention, the metal powder composition comprises primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten, a lubricant and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions. Preferably, the primary metal particles comprise copper or aluminum, which may be alloyed with conventional alloying elements.

A second method of forming a metal part according to the invention comprises: (i) providing a metal powder composition comprising primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten, a lubricant and an organic acid; (ii) placing the metal powder composition within a die cavity; (iii) applying pressure to the metal powder composition contained within the die cavity to form a green compact; (iv) heating the green compact in a non-oxidizing atmosphere to delube the metal powder composition and cause the organic acid to react with an oxide of a metal on the primary metal particles and form an organic metal salt; and (v) heating the delubed green compact to a peak sintering temperature in a non-oxidizing atmosphere to decompose the organic metal salt into a base metal and/or a metal carbide and form the metal part. The conversion of the metal oxide on the surface of the primary metal particles to an organic metal salt during the delubing step creates a “clean” surface on the primary metal particles that is receptive to both liquid phase bonding and subsequent diffusion bonding. Because no liquid phase forming materials or precursors thereof are present in the composition, the heating rate during sintering is not critical.

Metal parts formed using the metal powder compositions and methods according to the invention exhibit a substantially higher sintered density than metal parts formed from metal powder compositions that do not comprise an organic acid, and such higher densities can be reached in less time and at lower energy costs. For example, it is possible to form carbon steel or low alloy steel metal parts that have a sintered density that approaches 100% of theoretical density. Subsequent heat treatment of metal parts formed from the metal powder compositions and methods of the invention substantially improve the mechanical properties of the parts, which in some cases are better than can be achieved using non-powder metallurgical processes such as forging and casting.

The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.

DETAILED DESCRIPTION OF THE INVENTION

In a first embodiment of the invention, metal powder compositions according to the present invention comprise a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof, a lubricant and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions. Throughout the instant specification and in the accompanying claims, the term “primary metal particles” refers to the principal metal powder component of the metal powder composition by weight. The primary metal particles can comprise a single metallic element, or can be alloys, agglomerations or blends of two or more metallic elements. Suitable metals include, for example, iron, copper, chromium, aluminum, nickel, bismuth, cobalt, manganese, niobium, titanium, molybdenum, tin and tungsten. Iron is a particularly preferred metal and is the major constituent of steel.

The primary metal particles tend to have surfaces that are oxidized, typically as a result of contact with oxygen in the atmosphere or with water vapor. Primary metal particles comprising iron, which are frequently used in pressed powder metallurgy to form steel parts, have surfaces that are oxidized to form iron oxide. Applicants believe that metal oxides on the surface of primary metal particles may interfere with solid-state diffusion bonding between such particles during sintering. The metal oxides on the surface of the primary metal particles may also inhibit the formation of liquid phase alloys, which can be used to solder, weld or otherwise bind the individual metal particles together.

A variety of organic acids are known to react with metal oxides to produce organic metal salts. For example, acetic acid will react with iron oxide to form ferrous acetate. Similarly, citric acid will react with iron oxide to form ferrous citrate. Lactic acid will react with iron oxide to ferrous lactate. And, malic acid, tartaric acid, oxalic acid, oleic acid, and stearic acid will react with iron oxide to form ferric malate, ferrous tartrate, ferrous oxalate, ferric oleate and ferrous stearate, respectively.

Organic acids suitable for use in the invention are those which are strong enough to react with metal oxides on the surface of the primary metal particles to produce metal salts, and which are compatible with the mixing, filling and compaction and sintering steps of the pressed powder metallurgy process. Preferably, the organic acid or acids used in the invention do not leave undesirable residues or by-products when decomposed during delubing and sintering. Accordingly, organic acids that are free of, or contain very little, sulfur, nitrogen, phosphorous and halogens are preferred.

Fatty acids are particularly suitable organic acids for use in the invention. A non-exhaustive list of fatty acids is set forth in Section 7-28 (“Properties of Selected Fatty Acids”) of the CRC Handbook of Chemistry and Physics, 76th Edition (1995), which is hereby incorporated by reference. It will be appreciated that other organic acids can also be used. Many organic acids are listed in Section 8-45 to 8-55 (“Dissociation Constants of Organic Acids and Bases”) of the CRC Handbook of Chemistry and Physics, 76th Edition (1995), which is also hereby incorporated by reference. The organic acids identified in that list that are compatible with pressed powder metallurgy and which are free of, or contain very little, sulfur, nitrogen, phosphorous and halogens can be used.

Citric acid is the presently most preferred organic acid for use with metal powder compositions for low alloy steel and carbon steels as well as stainless steel, copper and aluminum. Other particularly useful organic acids include acids that have a pKA value low enough to react with metal oxides and which are solids at press conditions (typically ˜140° F. and higher). Examples of suitable alternative acids to citric acid include, for example, oxalic acid, tartaric acid, malic acid and low-melting acids that are partially solublized in higher melting acids or other organic materials that decompose into constituents that are similar to citric acid or the other acids identified above.

The amount of organic acid present in the metal powder composition will depend on the amount of metal oxide to be removed, and the ability of the organic acid to remove the metal oxide during the delubing/sintering cycle(s). Loadings from about 0.1% by weight to about 4% by weight are typically sufficient. Ideally, a stoichiometric amount of acid would be added relative to the oxides on the surface of the metal particles, plus an excess of about 10 mole percent, if press conditions would allow it. To insure adequate distribution of the organic acid in the metal powder composition, it is preferable that the organic acid be micronized to an average particle size of about 30 μm or less (e.g., via milling). When used neat (i.e., not blended with other materials), it is preferable for the organic acid to be mirconized close in time prior to use so that the micronized particles do not have an opportunity to degrade upon exposure to atmospheric moisture.

In the most preferred embodiment of the invention, the organic acid is used in combination with a lubricant (e.g., by creating a masterbatch comprising a blend of the lubricant, the organic acid and, optionally, other components of the powder metal composition such as graphite). Conventional lubricants such as ethylene bis-stearamide wax and zinc stearate can be used, but the lubricant described in U.S. Pat. No. 6,679,935, which is hereby incorporated by reference, is most preferred. Such a lubricant transforms from a solid to a liquid due to shear in the press, spreads and makes a uniform coating of lubricant, liquid phase forming materials and/or precursors and organic acid on the surface of the primary metal particles. The lubricant, due to its liquid nature, becomes less viscous as the temperature rises, and the molten lubricant can serve as an effective vehicle or solvent for the organic acid and the liquid phase forming materials and/or precursors thereof. It will be appreciated that some organic acids, particularly longer chain fatty acids, can serve as both a lubricant and a compound that assists in the removal of metal oxides from the surface of the metal powder particles.

Throughout the instant specification and in the accompanying claims, the term “liquid phase forming materials” refers to metallic alloys that, when present between adjacent primary metal particles in a liquid (molten) state during sintering, assist in forming a liquid phase bond (e.g. solder/weld-type bonds) between the primary metal particles. Liquid phase forming materials are separate and distinct from the primary metal particles, and are blended therewith to form a substantially homogeneous composition. Iron is the predominant metallic constituent of low alloy dry powder steel metal compositions, and the presence of carbon, manganese, silicon, phosphorous, sulfur, boron, chromium, cobalt and/or molybdenum on the surface of the oxide-free metal particles can lead to the liquid phase forming materials such as, for example, Fe—C—Mn, Fe—C, Fe—C—Si, Fe—Mn, Fe—P, Fe—S, Co—C, Mo—C, Mn—C, Ni—C, Fe—B and Fe—Cr. Precursors to liquid phase forming materials thus include graphite, ferro phosphorous, copper phosphorous, boron, silica, manganese sulphide, manganese, silicon, phosphorous, sulfur, boron, chromium, cobalt and/or molybdenum. Liquid phase forming materials and precursors thereof are conventionally used in powder metallurgy. The presence of an organic acid in compositions according to the first embodiment of the invention and the rapid heating of the delubed green compact during sintering allows for the use of reduced amounts of the liquid phase forming materials to achieve parts having higher sintered density, which can approach theoretical density.

The metal powder compositions according to the invention can be processed using conventional dry powder metal techniques. The powder metal composition is typically placed into a cavity and pressed to form a green part. The green part is then heated to remove the lubricant during a “delubing” step. The present compositions can be “delubed” using conventional delubing techniques. Delubing should be conducted in a non-oxidizing atmosphere.

As the green compact is heated during delubing, the organic acid present in the metal powder compositions according to the invention reacts with the metal oxides on the surface of the primary metal particles to form organic metal salts. Without being bound to a particularly theory, applicants believe that any one or more of three distinct reaction mechanisms may occur during the heating of the green compact, which facilitate the removal of the metal oxide layer from the surface of the primary metal particles: melt fusion; ionic; and/or vapor. In the melt fusion reaction mechanism, the organic acid would melt and boil on the surface of the primary metal particles, reaching temperatures that allow for a direct neutralization reaction. In the ionic reaction mechanism, the organic acid would partially dissolve in residual water that is bonded or adhered to the surface of the primary metal particles forming a hot ionic acid that dissolves the metal oxide as the temperature rises. In the vapor reaction mechanism, the organic acid would become volatile and scavenges the metal oxide layer as it escapes from the green compact.

Although the exact mechanism of the reaction between the organic acid and the metal oxide on the surface of the primary metal particles is not definitively known at present, applicants believe that the organic acid effectively removes all or some part of the metal oxides from the surface of the primary metal particles. The “cleaned” surfaces of adjacent primary metal particles are in contact with each other, which allows for better necking in the solid phase, because there is less hindrance or interference to diffusion bonding caused by the presence of a metal oxide at the interface between the particles. Applicants believe that some localized liquid phase sintering also probably occurs (even in the absence of liquid phase forming materials or precursors thereof), because the non-oxidized surfaces of adjacent metal particles are more reactive.

The iron oxide content of most commercial low alloy steel metal powder compositions for pressed powder metallurgy ranges from 0.05% to 0.5% by weight. Metal powders having the lowest oxygen content provide the best compressibility and best final properties, but are generally more expensive. Use of an organic acid according to the present invention allows for the removal of the oxygen from such metal particles, which is present as iron oxide. The organic acid reacts with the iron oxide or other metals to form an organic iron salt, which decomposes during sintering to form very finely divided iron metal or other base starting metals, which can serve to promote solid state sintering and localized liquid phase sintering, or iron carbide, which can be a component of the low alloy or carbon steel part. Thus, the present invention provides two distinct benefits: metal particles having surfaces that have all or some of metal oxides removed, which enhances the efficiency of both solid state and liquid phase sintering; and a by-product from the decomposition of the iron salt, which also enhances the solid state or liquid phase sintering.

Applicants have discovered that it is critical that the delubed compact be heated to the peak sintering temperature in a reducing atmosphere or inert atmosphere at a rate of about 60° F./min or more in order to obtain a metal part having a higher sintered density than would otherwise be obtained using a conventional metal powder composition that did not comprise an organic acid. Applicants believe that the delubing procedure removes all or part of the oxide layer from the surface of the metal particles at the last possible moment before sintering, which promotes solid-state diffusion and liquid phase sintering. Heating at a rate lower than 60° F./min does not appear to provide any improvement in sintered density.

Applicants theorize that once the metal oxides have been removed from the surface of the primary metal particles, the material present at or on the surface of the metal particles become highly receptive to solid state diffusion. If the heating rate is slow, diffusion occurs over an extended period of time contemporaneous with the relatively slow heating rate, allowing the material present at or on the surface of the particles time to diffuse into the particles, which depletes the amount of liquid phase forming material present on the surface of the particles to obtain liquid phase soldering, welding or bonding between the particles. In essence, a slow heating rate assures that bonding is accomplished predominantly or entirely by solid state diffusion, and not by liquid phase bonding. Use of a faster heating rate, on the other hand, reduces the time the liquid phase forming material at or on the surface of the cleaned particles has to diffuse into the particles, and thereby maintains sufficient amounts of liquid phase forming material to promote liquid phase bonding between the particles during the heating cycle. Liquid phase bonding is similar to soldering or welding, and leads to substantial improvements in the final density of the sintered parts. Thus, the rapid heating rate is necessary to provide sufficient time for liquid phase forming materials to form liquid-type bonding between the primary metal particles. The time period during which the rapid heating occurs may vary according to the particular heating process and equipment being used, but is typically accomplished within about ten minutes or less. High oven temperatures can be used (i.e., oven temperatures of as high as about 2,650° F., which is in excess of the melting temperature of the primary metal particles) so long as the metal part is not allowed to reach a temperature above the melting temperature of the primary metal particles. Use of sintering temperatures below the melting temperature of the primary metal particles can allow for extended dwell times, provided the heating rate is rapid. Sintering is typically conducted in a non-oxidizing, preferably reducing, atmosphere such as that which comprises a blend of hydrogen and nitrogen, or in endothermic (e.g. CO—H₂—N₂) or inert atmospheres (e.g., Ar).

The first method of forming a metal part according to the invention comprises: (i) providing a metal powder composition comprising a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof, a lubricant and an organic acid; (ii) placing the metal powder composition within a die cavity; (iii) applying pressure to the metal powder composition contained within the die cavity to form a green compact; (iv) heating the green compact in a non-oxidizing atmosphere to delube the metal powder composition and cause the organic acid to react with an oxide of a metal on the primary metal particles and form an organic metal salt; and (v) heating the delubed green compact to a peak sintering temperature at a heat up rate of 60° F./min or higher in a non-oxidizing atmosphere to decompose the organic metal salt into a base metal and/or a metal carbide and form the metal part. The conversion of the metal oxide on the surface of the primary metal particles to an organic metal salt during the delubing step creates a “clean” surface on the primary metal particles that is receptive to both liquid phase bonding and subsequent diffusion bonding. The rapid heating rate during the sintering step ensures that the liquid phase formers have adequate time to create liquid phase bonds between the primary metal particles before the constituents of the liquid phase diffuse into the particles. With more efficient oxide reduction or removal, leaner compositions reach higher densities. These leaner compositions have a smaller time window to react, which is made available by having an earlier removal of oxides.

In a second embodiment of the invention, the metal powder composition comprises primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten, a lubricant and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions. Preferably, the primary metal particles comprise copper or aluminum, which may be alloyed with conventional alloying elements. No liquid phase forming materials or precursors thereof are present in the composition according to the second embodiment of the invention. However, due to the low viscosity of the metal in the primary metal particles, the particles tend to fuse together, likely through diffusion alone, and form high density parts upon sintering. The absence of an oxide layer, which is stripped and converted to a metal salt during a delube step, yields primary metal particles having very “clean” (i.e., oxide-free or having very low amounts of oxide residues) surfaces, which are capable of bonding and fusing together without the need for liquid phase forming materials or precursors thereof.

Thus, a second method of forming a metal part according to the invention comprises: (i) providing a metal powder composition comprising primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten, a lubricant and an organic acid; (ii) placing the metal powder composition within a die cavity; (iii) applying pressure to the metal powder composition contained within the die cavity to form a green compact; (iv) heating the green compact in a non-oxidizing atmosphere to delube the metal powder composition and cause the organic acid to react with an oxide of a metal on the primary metal particles and form an organic metal salt; and (v) heating the delubed green compact to a peak sintering temperature in a non-oxidizing atmosphere to decompose the organic metal salt into a base metal and/or a metal carbide and form the metal part. The conversion of the metal oxide on the surface of the primary metal particles to an organic metal salt during the delubing step creates a “clean” surface on the primary metal particles that is receptive to both liquid phase bonding and subsequent diffusion bonding. Because no liquid phase forming materials or precursors thereof are present in the composition, the heating rate during sintering is not critical.

Metal parts formed using the metal powder compositions and methods according to the invention exhibit a substantially higher sintered density than metal parts formed from metal powder compositions that do not comprise an organic acid, and such higher densities can be reached in less time and at lower energy costs. For example, it is possible to form carbon steel or low alloy steel metal parts that have a sintered density that approaches 100% of theoretical density. Copper parts can also be formed in accordance with the invention that have sintered densities approaching 100% of theoretical density. Subsequent heat treatment of metal parts formed from the metal powder compositions and methods of the invention substantially improve the mechanical properties of the parts, which in some cases are better than can be achieved using non-powder metallurgical processes such as forging and casting.

The following examples are intended only to illustrate the invention and should not be construed as imposing limitations upon the claims.

EXAMPLE 1

A Stock Powder Metallurgy Composition (“Stock P/M”) was prepared by dry mixing the components set forth in Table 1 below: TABLE 1 Component Weight Percent ANCORSTEEL 85 HP* 97.00%  UT-3PM** 2.00% Graphite Powder 0.65% SUPERLUBE PS1000-B*** 0.35% *ANCORSTEEL 85 HP is a water atomized, pre-alloyed steel powder (approximate chemical composition in weight percent: ˜98.93% Fe; 0.86% Mo; 0.12% Mn; 0.08% O; and <0.1% C)available from Hoeganeaes Corporation of Cinnaminson, New Jersey. **UT-3PM is a high-purity nickel powder for pressed powder metallurgy applications available from Norilsk of Moscow, Russia. ***SUPERLUBE PS1000-B is a pressed powder metallurgy lubricant capable of transforming from a solid to a liquid due to shear from Apex Advanced Technologies of Cleveland, Ohio.

EXAMPLE 2

Test bars were formed using the Stock P/M formed in Example 1. In Sample 1, the test bar was formed solely out of the Stock P/M formed in Example 1. In Samples 2 and 3, the test bars were formed by blending the Stock P/M with citric acid at a 0.2% by weight loading and a 0.4% by weight loading, respectively. Each test bar was formed using a 50 tsi (tons per square inch) Tinius Olsen hydraulic press. Each test bar had the following dimensions: ½″ wide×1¼″ long×¼″ thick.

The green density of the pressed test bars was measured in accordance with the procedures set forth in MPIF Standard 45 and ASTM B331-95 (2002). The green test bars were delubed at normal conditions and were sintered in a continuous furnace at a heat up rate of 133° F./min in the hot zone to a temperature of 2,480° F. in an atmosphere consisting of 25% H₂ and 75% N₂. The density of the green and sintered test bars is reported in Table 2 below: TABLE 2 Sample Stock P/M Citric Acid Green Density Sintered Density 1  100%  0% 7.24 g/cm³ 7.32 g/cm³ 2 99.8% 0.2% 7.15 g/cm³ 7.81 g/cm³ 3 99.6% 0.4% 7.11 g/cm³ 7.83 g/cm³

The data reported in Table 2 shows that at a high heat up rate (>60° F./min), the presence of a small amount of citric acid in the Stock P/M blend results in a substantial improvement in sintered density. Specifically, the data in Table 2 shows that blending 0.4% by weight of citric acid with the Stock P/M coupled with a heat up rate of 133° F./min increases the sintered density of the test bars from 7.32 g/cm³ to 7.83 g/cm³, which is an improvement from 93.25% to 99.75% of theoretical density.

EXAMPLE 3

The test bars were formed using the same Stock P/M formed in Example 1 using the same procedures as set forth in Example 2. The green test bars were delubed at normal conditions, sintered in a continuous furnace at a heat up rate of 50° F./min in the hot zone to a temperature of 2,480° F. in an atmosphere consisting of 25% H₂ and 75% N₂. The density of the green and sintered test bars is reported in Table 3 below: TABLE 3 Sample Stock P/M Citric Acid Green Density Sintered Density 4  100%  0% 7.29 g/cm³ 7.42 g/cm³ 5 99.6% 0.4% 7.21 g/cm³ 7.35 g/cm³ 6 99.2% 0.8% 7.10 g/cm³ 7.23 g/cm³

The data reported in Table 3 shows that the presence of small amounts of citric acid in the Stock P/M blend does not result in any improvement in sintered density when the heat up rate is below 60° F./min. Specifically, the sintered density of the test bars decreased with the addition of citric acid at a heat up rate of 50° F./min due to lower green density to start. Typically there is a direct correlation between green densities and sintered, the lower it starts the lower it goes.

EXAMPLE 4

Test bars were formed using the same Stock P/M formed in Example 1 using the same procedures as set forth in Example 2. The green test bars were delubed at normal conditions, sintered in a continuous furnace at a heat up rate of 15° F./min in the hot zone to a temperature of 2,460° F. in an atmosphere consisting of 25% H₂ and 75% N₂. The density of the green and sintered test bars is reported in Table 4 below: TABLE 4 Sample Stock P/M Citric Acid Green Density Sintered Density 7  100%  0% 7.29 g/cm³ 7.43 g/cm³ 8 99.6% 0.4% 7.27 g/cm³ 7.46 g/cm³ 9 99.2% 0.8% 7.11 g/cm³ 7.45 g/cm³

The data reported in Table 4 shows that the presence of small amounts of citric acid in the Stock P/M blend had no appreciable effect on the sintered density at conventional powder metallurgy heat up rates. Specifically, the sintered density of the test bars was relatively constant with the addition of citric acid at a heat up rate of 15° F./min.

EXAMPLE 5

The Stock P/M Composition from Example 1 was used to form test bars as described in Example 2. One set of test bar samples were pressed solely out of the Stock P/M Composition. A second set of test bar samples were pressed out of the Stock P/M Composition mixed with an additional 0.4% by weight of citric acid. All of the test bars were delubed in a continuous furnace in an inert atmosphere consisting of 100% nitrogen at a peak temperature below about 410° F. at a heating rate of about 16° F. per minute. The test bars were then allowed to cool to ambient temperature (˜72° F.) and later were placed in a microwave furnace under a reducing atmosphere and heated for 2.5 minutes. The test bars that did not include citric acid reached a sintered density of 7.65 g/cm³ at 1356°F., whereas the test bars that did include citric acid reached a sintered density of 7.81g/cm³ at the same temperature. Theoretical density would be considered to be ˜7.82-7.84 g/cm³. The temperature noted is a reference temperature only. The actual part temperature may have been higher at the peak of heating. Rapid heating of the test bars that included an organic acid resulted in significantly higher sintered density than the test bars that did not include an organic acid.

EXAMPLE 6

A powder metal grade of powdered copper (Acupowder Grade 165: ˜99.5% purity) was mixed with 0.35% by weight of Apex Lubricant (PS1000b) and 0.1% by weight lithium stearate and pressed into test bars as described in Example 2. Lithium stearate is generally known and regarded in the art as an additive that helps copper achieve higher density. A second set of test bars were pressed out of a composition comprising the same powdered copper, 0.35% by weight of Apex Lubricant (PS1000b) and 0.4% by weight citric acid. All of the test bars were then delubed and sintered in one operation in a batch furnace at 15° F. degrees per minute in 100% hydrogen up to 1930° F. with a 30 minute hold-at temperature. Rapid heating after the delube step was not required to obtain higher sintered density because there were no alloying/liquid phase forming elements present in the composition. The test bars that did not include citric acid reached a sintered density of 8.05 g/cm³, whereas the test bars that did include citric acid reached a sintered density of 8.95 g/cm³. Theoretical density ranges from 8.92 to 8.96. By removal of the surface oxides alone the density achieved 100% theoretical.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and illustrative examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

1. A method of forming a metal part comprising the steps of: (i) providing a metal powder composition comprising a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof and an organic acid; (ii) placing the metal powder composition within a die cavity; (iii) applying pressure to the metal powder composition contained within the die cavity to form a green compact; (iv) heating the green compact in a non-oxidizing atmosphere to delube the metal powder composition and cause the organic acid to react with an oxide of a metal on the primary metal particles and form an organic metal salt; and (v) heating the delubed green compact to a peak sintering temperature at a heat up rate of 60° F./min or higher in a non-oxidizing atmosphere to decompose the organic metal salt into a base metal and/or a metal carbide and form the metal part.
 2. The method according to claim 1 wherein the metal powder composition further comprises a lubricant.
 3. The method according to claim 1 wherein the heating steps (iv) and (v) are conducted in a reducing atmosphere.
 4. The method according to claim 1 wherein heating step (v) is conducted in a vacuum furnace, a continuous furnace or a microwave furnace.
 5. The method according to claim 1 wherein the organic acid is free of sulfur, halogens, nitrogen and phosphorous.
 6. The method according to claim 1 wherein the organic acid is a fatty acid.
 7. The method according to claim 1 wherein the organic acid is present in the metal powder composition at a loading of from about 0.1% to about 4.0% by weight.
 8. The method according to claim 1 wherein the primary metal particles comprise one or more metals selected from the group consisting of iron, copper, chromium, aluminum, nickel, bismuth, cobalt, manganese, niobium, titanium, molybdenum, tin and tungsten.
 9. The method according to claim 1 wherein the liquid phase forming materials are selected from the group consisting of Fe—C—Mn, Fe—C, Fe—C—Si, Fe—Mn, Fe—P, Fe—S, Co—C, Mo—C, Mn—C, Ni—C, Fe—B and Fe—Cr.
 10. The method according to claim 1 wherein the metal powder composition comprises precursors of liquid phase forming materials selected from the group consisting of graphite, ferro phosphorous, copper phosphorous, boron, silica, manganese sulphide, manganese, silicon, phosphorous, sulfur, boron, chromium, cobalt and/or molybdenum.
 11. The method according to claim 1 wherein the primary metal particles comprise iron and the organic acid comprises citric acid.
 12. The method according to claim 3 wherein the reducing atmosphere comprises a mixture of hydrogen and nitrogen or vacuum with or without partial pressure.
 13. A metal part formed according to the method of claim
 1. 14. The metal part according to claim 13 wherein the metal part comprises a carbon or low-alloy steel having a sintered density of greater than 95% of theoretical density.
 15. A metal powder composition for use in pressed powder metallurgy comprising a blend of primary metal particles, one or more liquid phase forming materials or precursors thereof and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions.
 16. The metal powder composition according to claim 15 further comprising a lubricant.
 17. The metal powder composition according to claim 15 wherein the organic acid is free of sulfur, halogens, nitrogen and phosphorous.
 18. The metal powder composition according to claim 15 wherein the organic acid is a fatty acid.
 19. The metal powder composition according to claim 15 wherein the organic acid is present in the metal powder composition at a loading of from about 0.1% to about 4.0% by weight.
 20. The metal powder composition according to claim 15 wherein the primary metal particles comprise one or more metals selected from the group consisting of iron, copper, chromium, aluminum, nickel, bismuth, cobalt, manganese, niobium, titanium, molybdenum, tin and tungsten.
 21. The metal powder composition according to claim 15 wherein the liquid phase forming materials are selected from the group consisting of Fe—C—Mn, Fe—C, Fe—C—Si, Fe—Mn, Fe—P, Fe—S, Co—C, Mo—C, Mn—C, Ni—C, Fe—B and Fe—Cr.
 22. The metal powder composition according to claim 15 wherein the metal powder composition comprises precursors of liquid phase forming materials selected from the group consisting of graphite, ferro phosphorous, copper phosphorous, boron, silica, manganese sulphide, manganese, silicon, phosphorous, sulfur, boron, chromium, cobalt and/or molybdenum.
 23. The metal powder composition according to claim 15 wherein the primary metal particles comprise iron and the organic acid comprises citric acid.
 24. The metal powder composition according to claim 15 wherein the reducing atmosphere comprises a mixture of hydrogen and nitrogen or vacuum with or without partial pressure.
 25. A metal part formed of the metal powder composition according to claim
 15. 26. The metal part according to claim 25 wherein the metal part comprises a carbon or low-alloy steel having a sintered density of greater than 95% of theoretical density.
 27. A method of forming a metal part according to the invention comprises: (i) providing a metal powder composition comprising primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten and an organic acid; (ii) placing the metal powder composition within a die cavity; (iii) applying pressure to the metal powder composition contained within the die cavity to form a green compact; (iv) heating the green compact in a non-oxidizing atmosphere to delube the metal powder composition and cause the organic acid to react with an oxide of a metal on the primary metal particles and form an organic metal salt; and (v) heating the delubed green compact to a peak sintering temperature in a non-oxidizing atmosphere to decompose the organic metal salt into a base metal and/or a metal carbide and form the metal part.
 28. The method of forming a metal part according to claim 27 wherein the metallic elements having relatively low viscosity when molten in the primary metal particles are selected from the group consisting of copper and aluminum.
 29. The method according to claim 27 wherein the metal powder composition further comprises a lubricant.
 30. A metal powder composition comprising primary metal particles comprising a major amount of one or more metallic elements having relatively low viscosity when molten and an organic acid that is capable of reacting with an oxide of a metal in the primary metal particles to form an organic metal salt that decomposes when the metal powder composition is sintered under reducing or non-oxidizing conditions.
 31. The metal powder composition according to claim 30 wherein the metallic elements having relatively low viscosity when molten in the primary metal particles are selected from the group consisting of copper and aluminum.
 32. The metal powder composition according to claim 30 further comprising a lubricant. 